Optical transmission apparatuses, methods, and systems

ABSTRACT

Apparatuses, methods, and systems are disclosed for controlling optical signal wavelength spacing by providing for simultaneous upconversion of a plurality of electrical signal on subcarrier frequencies of an optical carrier frequency with or without modulation of an electrical data signal onto the optical carrier frequency. The optical carrier lightwave is split into a plurality of split lightwaves upon which one or more electrical frequencies carrying information can be upconverted onto optical subcarriers of the lightwave carrier frequency. The relative spacings of the optical subcarrier lightwaves will thus be unaffected by variation in the carrier frequency. The optical subcarrier lightwaves can then be recombined to form the optical data signal carrying the plurality of information carried by the electrical frequencies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 09/185,816, filed Nov. 4, 1998, now U.S. Pat. No.6,292,898, which is incorporated herein by reference, and which isrelated to commonly assigned U.S. patent application Ser. No.09/185,821, now abandoned entitled “Optical Distortion CompensationApparatuses, Methods, and Systems”, and 09/185,820, now U.S. Pat. No.6,118,566, entitled “Optical Upconverter Apparatuses, Methods, andSystems”, which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

The present invention is directed generally to the transmission ofinformation in communication systems. More particularly, the inventionrelates to transmitting information via optical signals in opticaltransmission systems and transmitters for use therein.

The development of digital technology provided resources to store andprocess vast amounts of information. While this development greatlyincreased information processing capabilities, it was soon recognizedthat in order to make effective use of information resources, it wasnecessary to interconnect and allow communication between informationresources. Efficient access to information resources requires thecontinued development of information transmission systems to facilitatethe sharing of information between resources.

The continued advances in information storage and processing technologyhas fueled a corresponding advance in information transmissiontechnology. Information transmission technology is directed towardproviding high speed, high capacity connections between informationresources. One effort to achieve higher transmission capacities hasfocused on the development of optical transmission systems for use inconjunction with high speed electronic transmission systems. Opticaltransmission systems employ optical fiber networks to provide highcapacity, low error rate transmission of information over long distancesat a relatively low cost.

The transmission of information over fiber optic networks is performedby imparting the information in some manner to a lightwave carrier byvarying the characteristics of the lightwave. The lightwave is launchedinto the optical fiber in the network to a receiver at a destination forthe information. At the receiver, a photodetector is used to detect thelightwave variations and convert the information carried by thevariations into electrical form.

In most optical transmission systems, the information is imparted byusing the information data stream to either modulate a lightwave sourceto produce a modulated lightwave or to modulate the lightwave after itis emitted from the light source. The former modulation technique isknown as “direct modulation”, whereas the latter is known as “externalmodulation”, i.e., external to the lightwave source. External modulationis more often used for higher speed transmission systems, because thehigh speed direct modulation of a source often causes undesirablevariations in the wavelength of the source. The wavelength variations,known as chirp, can result in transmission and detection errors in anoptical system.

Data streams can be modulated onto the lightwave using a number ofdifferent schemes. The two most common schemes are return to zero (RZ)and non-return to zero (NRZ). In RZ modulation, the modulation of eachbit of information begins and ends at the same modulation level, i.e.,zero, as shown in FIG. 1(a). In NRZ schemes, the modulation level is notreturned to a base modulation level, i.e., zero, at the end of a bit,but is directly adjusted to a level necessary to modulate the nextinformation bit as shown in FIG. 1(b). Other modulation schemes, such asduobinary and PSK, encode the data in a waveform, such as in FIG. 1(c),prior to modulation onto a carrier.

In many systems, the information data stream is modulated onto thelightwave at a carrier wavelength, λ_(c), (FIG. 2(a)) to produce anoptical signal carrying data at the carrier wavelength, similar to thatshown in FIG. 2(b). The modulation of the carrier wavelength alsoproduces symmetric lobes, or sidebands, that broaden the overallbandwidth of the optical signal. The bandwidth of an optical signaldetermines how closely spaced successive optical signals can be spacedwithin a range of wavelengths.

Alternatively, the information can be modulated onto a wavelengthproximate to the carrier wavelength using subcarrier modulation (“SCM”).SCM techniques, such as those described in U.S. Pat. Nos. 4,989,200,5,432,632, and 5,596,436, generally produce a modulated optical signalin the form of two mirror image sidebands at wavelengths symmetricallydisposed around the carrier wavelength. Generally, only one of themirror images is required to carry the signal and the other image is asource of signal noise that also consumes wavelength bandwidth thatwould normally be available to carry information. Similarly, the carrierwavelength, which does not carry the information, can be a source ofnoise that interferes with the subcarrier signal. Modified SCMtechniques have been developed to eliminate one of the mirror images andthe carrier wavelength, such as described in U.S. Pat. Nos. 5,101,450and 5,301,058.

Initially, single wavelength lightwave carriers were spatially separatedby placing each carrier on a different fiber to provide space divisionmultiplexing (“SDM”) of the information in optical systems. As thedemand for capacity grew, increasing numbers of information data streamswere spaced in time, or time division multiplexed (“TDM”), on the singlewavelength carrier in the SDM system as a means to provide additionalcapacity. The continued growth in transmission capacity has spawned thetransmission of multiple wavelength carriers on a single fiber usingwavelength division multiplexing (“WDM”). In WDM systems, furtherincreases in transmission capacity can be achieved not only byincreasing the transmission rate of the information via each wavelength,but also by increasing the number of wavelengths, or channel count, inthe system.

There are two general options for increasing the channel count in WDMsystems. The first option is to widen the transmission bandwidth to addmore channels at current channel spacings. The second option is todecrease the spacing between the channels to provide a greater number ofchannels within a given transmission bandwidth. The first optioncurrently provides only limited benefit, because most optical systemsuse erbium doped fiber amplifiers (“EDFAs”) to amplify the opticalsignal during transmission. EDFAs have a limited bandwidth of operationand suffer from non-linear amplifier characteristics within thebandwidth. Difficulties with the second option include controllingoptical sources that are closely spaced to prevent interference fromwavelength drift and nonlinear interactions between the signals.

A further difficulty in WDM systems is that chromatic dispersion, whichresults from differences in the speed at which different wavelengthstravel in optical fiber, can also degrade the optical signal. Chromaticdispersion is generally controlled in a system using one or more ofthree techniques. One technique to offset the dispersion of thedifferent wavelengths in the transmission fiber through the use ofoptical components such as Bragg gratings or arrayed waveguides thatvary the relative optical paths of the wavelengths. Another technique isintersperse different types of fibers that have opposite dispersioncharacteristics to that of the transmission fiber. A third technique isto attempt to offset the dispersion by prechirping the frequency ormodulating the phase of the laser or lightwave in addition to modulatingthe data onto the lightwave. For example, see U.S. Pat. Nos. 5,555,118,5,778,128, 5,781,673 or 5,787,211. These techniques require thatadditional components be added to the system and/or the use of specialtyoptical fiber that has to be specifically tailored to each length oftransmission fiber in the system.

New fiber designs have been developed that substantially reduce thechromatic dispersion of WDM signals during transmission in the 1550 nmwavelength range. However, the decreased dispersion of the opticalsignal allows for increased nonlinear interaction, such as four wavemixing, to occur between the wavelengths that increases signaldegradation. The effect of lower dispersion on nonlinear signaldegradation becomes more pronounced at increased bit transmission rates.

The many difficulties associated with increasing the number ofwavelength channels in WDM systems, as well as increasing thetransmission bit rate have slowed the continued advance incommunications transmission capacity. In view of these difficulties,there is a clear need for transmission techniques and systems thatprovide for higher capacity, longer distance optical communicationsystems.

BRIEF SUMMARY OF THE INVENTION

Apparatuses and methods of the present invention address the above needby providing optical communication systems that include transmittersthat can provide for pluralities of information carrying wavelengths peroptical transmission source, dispersion compensation, and/or nonlinearmanagement in the system. In an embodiment, the information data streamis electrically distorted to compensate for chromatic dispersion of alightwave/optical signal during transmission. The electrical distortioncan be used to compensate for negative or positive dispersion in varyingamounts depending upon the characteristics of the optical fiber in thenetwork and to some extent offset nonlinear interactions that producedistortion of the optical signal Electrical distortion can bespecifically tailored to each wavelength and bit rate used in theoptical system.

Electrical dispersion compensation can be used in conjunction with othermethods, such as dispersion compensating fiber or time delay componentsto control the level of dispersion at various points in the network. Theamount of dispersion in the system can be controlled to provide asubstantially predetermined value of net dispersion, e.g., zero, at theend of a link, to provide an average value over the link, and/or tominimize the absolute dispersion at any point in the link.

Electrical distortion compensation can be used with RZ, NRZ, ASK, PSK,and duobinary formats, as well as other modulation formats and basebandand subcarrier modulation techniques. In addition, the amount ofelectronic distortion applied to a signal can be controlled via afeedback loop from a receiver in the system to allow fine tuning of thecompensation. In this manner, changes in the network performance withtime can be accommodated.

In an embodiment, an information data stream is modulated on to anelectrical carrier, such radio frequency (“RF”) or microwave carrier,frequency ν_(e). The modulated electrical carrier is upconverted on to alightwave carrier having a wavelength λ₀ and frequency ν_(o) produced bythe optical transmission source to produce an information carryinglightwave at wavelength λ₁ and frequency ν_(o±e). The upconverter can beused to simultaneously upconvert a plurality of electrical frequenciesonto different subcarrier lightwaves. In an embodiment, the informationis modulated onto the electrical carrier in duobinary format, whichprovides for more narrow subcarrier bandwidths.

In an embodiment, the lightwave carrier from the optical source is splitinto a plurality of split lightwave carriers, each of which has one ormore data streams upconverted or modulated onto it. The subcarrierlightwave optical signals generated from the split lightwave opticalcarriers are then recombined into the optical signal for transmission.The split lightwave carrier overcomes the problem of maintaining closewavelength spacing between multiple carriers in an operating system byemploying a common optical source. The optical source providing thelightwave carrier may include one or more lasers or other opticalsources.

The split lightwave carrier also provides a method of placing multipleinformation carrying wavelengths near the lightwave carrier withouthaving to upconvert or modulate more than one data stream at a time ontoa lightwave carrier. The upconverted lightwaves can be at wavelengthsthat are greater and/or less than the carrier wavelength andsymmetrically or asymmetrically positioned relative to the carrierwavelength. In addition, subcarriers can be simultaneously upconvertedonto the same lightwave, at least one subcarrier with a higher frequencyand at least one subcarrier with a lower frequency than the carrierfrequency.

The upconversion of the modulated electrical carrier can be performedusing double or single sideband upconverters with or without suppressionof the carrier wavelength λ_(o). However, the reduction or eliminationof the carrier wavelength λ_(o) and any mirror image sideband willeliminate unwanted signals which could interfere with the upconvertedsignal.

In an embodiment, a two sided, single sideband upconverter is providedto modulate multiple information data streams onto both longer andshorter wavelengths. In those embodiments, one upconverter can be usedto upconvert data on equally or differently spaced subcarriers relativeto the carrier wavelength.

In an embodiment, the polarization of adjacent lightwave carriers iscontrolled to decrease the nonlinear interactions of the signals. Forexample, adjacent-wavelength signal can be orthogonally polarized todecrease the extent of four wave mixing that occurs between the signalsduring transmission. In addition, the wavelength spacing betweenneighboring upconverted signals can be selected to lessen non-linearinteraction effects.

Accordingly, the present invention addresses the aforementioned problemswith providing increasing the number of channels and the transmissionperformance of optical systems. These advantages and others will becomeapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings wherein likemembers bear like reference numerals and wherein:

FIGS. 1(a-c) show a typical baseband return to zero (“RZ”) andnon-return to zero (“NRZ”) data signal;

FIGS. 2(a-c) show the intensity versus wavelength plots for anunmodulated optical carrier, modulated carrier, and modulatedsubcarriers of the carrier;

FIGS. 3-4 show embodiments of the system of the present invention; and,

FIG. 5 shows an embodiment of a transmitter of the present invention;

FIGS. 6(a, b & c) show transmission & reception time versus wavelengthcurves;

FIGS. 7(a-c) show embodiments of signal distorters of the presentinvention

FIGS. 8-11 show embodiments of transmitters of the present invention

FIG. 12 shows an embodiment of transmitters of the invention;

FIG. 13 shows an embodiment of upconverters of the present invention;

FIGS. 14-16 show embodiments of transmitters of the present invention;and,

FIG. 17 shows a polarizing element of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The operation of optical systems 10 of the present invention will bedescribed generally with reference to the drawings for the purpose ofillustrating present embodiments only and not for purposes of limitingthe same. As shown in FIG. 3, the system 10 includes an opticaltransmitter 12 configured to transmit information, i.e., data, etc., viaone or more information carrying optical wavelengths λ_(i) to an opticalreceiver 14 through one or more segments of optical fiber 16 _(j). Thesystem 10 may also include one or more dispersion compensatingcomponents 18 and feedback controllers 20, as well as other opticalcomponents such as optical amplifier 22, add/drop devices 24, and thelike.

As shown in FIG. 4, the system 10 can be embodied as a network includinga plurality of transmitters 12 and receivers 14 in optical communicationthrough one or more optical switches 26, combiners 28, and/ordistributors 30. For example, optical and digital cross connect switchesand routers, multiplexers, splitters, and demultiplexers can be employedin the system 10. The transmitters 12 and receivers 14 can interfacedirectly with electrical transmission systems or via electrical switchesor interfaces to other optical systems that operate using the same ordifferent wavelengths.

In an embodiment, the transmitter 12 is configured to electricallydistort an electrical signal carrying data to compensate for chromaticdispersion that occurs as an optical signal Λ_(o) carrying the data istransmitted through the optical fiber 16 _(i). The electronic datasignal Λ_(E) can be in a baseband Λ_(B) (i.e., binary, direct current),coded Λ_(c), or a modulated electrical carrier Λ_(e) format.

In an embodiment of the transmitter 12 shown in FIG. 5, an electronicsignal distorter 32 is configured to produce a distorted electricalsignal Λ_(ED). A distorted optical signal Λ_(OD) is produced using anelectrical to optical converter 33 to impart the the electrical signalΛ_(ED) onto an optical carrier lightwave Λ_(o). The electrical tooptical conversion can be performed by upconverting the electricalsignal Λ_(ED) onto a subcarrier lightwave of an optical carrierlightwave Λ_(o) provided by an optical source 34. Alternatively, theconversion of electrical signal Λ_(ED) can be performed by directlymodulating the optical source 34 or externally modulating the opticalcarrier lightwave Λ_(o) to produce the optical data signal at thecarrier frequency. One or more signal lasers, or other appropriateoptical sources as may be known in the art, can be used as the opticalsource 34.

The distortion of the electronic data signal is generally in the form ofan electronically induced time delay that varies as a function of theoptical wavelength λ_(i) in the optical signal Λ_(o). The group delaycan be used to provide varying amounts of dispersion compensation foreach wavelength and for each bit rate in the system 10. The time delaycharacteristics can be controlled to provide linear and nonlinear, aswell as positive, negative, and varying delay profiles with respect tothe wavelength of the signal. FIG. 6(a) shows an example of a typicalrelative time delay at the receiver versus wavelength plot for anoptical signal being transmitted with zero dispersion at a transmissiontime t_(t). Dispersion of the signal during transmission results in thedifferent wavelengths in the signal reaching the receiver 14 atdifferent time during a reception time interval, Δt_(r). The time delayin signal reception is one source of signal distortion that degradessystem performance. In the present invention, distorted optical signalscan be produced by introducing distortion as a group delay function offrequency, which results in the signal being transmitted over atransmission time interval Δt_(t). The electronic distortion is offsetby dispersion in the transmission path resulting in the differentfrequencies reaching the receiver 14 at the same reception time t_(r)(FIG. 6(b)), or over a reception time interval of choice (FIG. 6(c)).

One skilled in the art will appreciate that in the present invention thedistortion profile of the electronic data signal can be varied asdesired to control the shape of optical signal at the receiver 14. Forexample, given the interrelation of chromatic dispersion and nonlinearinteractions, the electrical distortion characteristics can be shaped tominimize the total distortion at the receiver 14 as opposed tominimizing only the chromatic dispersion. In addition, electronicdispersion compensation can be used in conjunction with dispersioncompensating elements 18, such as negative dispersion slope fiber,grating-based elements, etc. as are known in the art.

FIGS. 7(a-c) show embodiments of signal distorter 32 of the presentinvention. In FIG. 7(a),the distorter 32 includes one or more serialelectrical circulators 36 having an input to an input port 1 thatcirculates the electrical signal to an equalizer port 2. A resonator 38can be connected to port 2 to serve as an all-pass transmission filterthat reflects all incident power in a frequency dependent manner back tothe port 2, thereby distorting signal. The distorted electrical signalΛ_(ED) exits an output port 3 of the circulator 36 from which it can bepassed into another distortion element or exit the signal distorter 32.

An example of resonators 38, which are suitable for use in the presentinvention are impedence resonators following the general equation:

Z(s)=sL+1/(sC)

L=RQ/(2πf ₀)

C=1/(4πf ₀ ² L)

H(s)=(Z(s)−R)/(Z(s)+R)

D(ω)=−d/dω(arg(H(jω))),

where

Z = impedance C = capacitance D(ω) = group delay L = inductance f₀ =frequency H(s) = equalizer R = resistance Q = Q factor Transfer function

One skilled in the art will appreciate that the circulator/resonatorembodiments shown in FIG. 7(a) can be cascaded to provide desired groupdelay characteristics and that other networks may be used in the presentinvention.

For example, in FIG. 7(b), the signal distorter 32 includes one or moreelectrical loop couplers 35 configured to introduce the desired groupdelay into the electical carrier signal Λ_(e). Various configurations ofloop couplers suitable to achieve the desired group delay can be used inthe distorter 32. FIG. 7(c) shows an embodiment of the signal distorter32 for distorting the baseband signal Λ_(B). The distorter 32 is used toseparate the baseband signal Λ_(B) into I and Q components byconfiguring the inductors 37 and capacitors 39 to approximate thefollowing general transfer function over the frequency range ofinterest:

|H _(I)(jω)|² +|H _(Q)(jω)|²=constant.

The amount of dispersion in optical fiber 16 _(i) is generally welldocumented as a function of fiber length and optical wavelength. Forexample, transmission fiber can typically be in the range of 15-20ps/nm/km in the 1550 nm wavelength range. Thus, the amount of distortionnecessary to produce a desired dispersion profile at a point in theoptical transmission system can be calculated and adjusted as may benecessary in the system 10. In addition, the shape of the distortionprofile can be tailored to be linear or nonlinear functions of frequencyto compensate for the interrelation of chromatic dispersion andnonlinear interactions.

FIG. 8 shows an embodiment of the transmitter 12 in which an electricalmodulator 40 is used to modulate the baseband electric signal Λ_(B) ontoan electrical carrier at a frequency ν_(e) from an electrical carriersource 42. The modulator 40 can be a double balanced mixer as is knownin the art. The electrical carrier signal ν_(e) will be of the generalform A(sin(ω+φ) and the baseband signal Λ_(E) of the form V(t) resultingin an output signal of the general form kV(t)A(sin(ω+φ+φ₁). Thus, if themean of the baseband signal is zero, the carrier frequency will besuppressed. Likewise, if V(t) has essentially two state ±a, the outputwill be in PSK format.

The electrical carrier frequency can be any suitable frequency for thedata rate being transmitted, for example, RF or microwave carriers. Thesignal distorter 32 receives the modulated electrical carrier signalΛ_(e) at frequency ν_(e) and provides the distorted electrical carriersignal Λ_(eD). An upconverter 44 combines the distorted modulatedelectrical carrier at ν_(e) with an optical lightwave carrier at acentral wavelength λ_(o) (frequency ν_(o)) supplied by an optical source34. The resulting distorted optical signal Λ_(OD) has a frequencyν_(O)±ν_(e) (“V_(o±e)”) and central wavelength at λ_(o±e), which isequal to c/(ν_(o)±ν_(e)), where c is the speed of light.

In embodiments shown in FIGS. 8(b) and 9, the baseband electrical signalΛ_(B) is provided to the signal distorter 32, which is configured toseparate the signal Λ_(B) into in-phase (“I”) and quadrature (“Q”)components and distort the signal. The IQ components of the distortedelectrical signal Λ_(BD) are provided to an IQ modulator 46. In the FIG.8(b) embodiments, the I and Q components are modulated onto theelectrical carrier ν_(e) which is upconverted onto the optical carrierν_(o) to produce the distorted optical signal Λ_(OD) at the centralwavelength at λ_(o±e). In FIG. 9 embodiments, the I and Q components aremodulated onto the optical carrier having a central wavelength λ_(o) andfrequency ν_(o) to provide the distorted optical signal Λ_(OD) havingthe same central wavelength at λ_(o).

Conversely in FIG. 10, the baseband signal Λ_(B) is modulated onto aportion of the electrical carrier ν_(e), which is passed through thesignal distorter 32 to produce the distorted electrical signal Λ_(eD).Another portion of the electrical carrier ν_(e) is provided as inputalong with the distorted electrical signal ν_(eD) to an IQ demodulator48, which separates the distorted electrical signal Λ_(eD) into its IQcomponents. The IQ components of the electronic signal are provided tothe IQ modulator 46 which modulates the data onto the optical carrier atthe central wavelength λ_(o) and frequency ν_(o) provided by the opticalsource 34.

In the transmitter 12 of FIG. 11, the electrical baseband signal Λ_(B)can be encoded along with a clock signal Λ_(CLK) using a data encoder 50to provide an encoded data signal Λ_(C). The encoded data signal Λ_(C)may be further passed through a filter 52, such as a low pass filter, toshape the signal before being passed to the signal distorter 32. In thetransmitter 12 of FIG. 11, the IQ modulator 46 can be used to modulatethe distorted electrical signal onto the electrical carrier frequencyν_(e). The electrical carrier can be amplified using an electricalamplifier 54, split through electrical coupler 56, and upconverted ontothe optical carrier to produce the distorted optical signal Λ_(OD)having its center wavelength at λ_(o±r). One of the controllers 20 inthe system 10 can be used to provide feedback control of the upconverter44, as well as the other components such as the amplifier 54.

In embodiments of FIG. 11, the electrical coupler 56 is used to splitthe signal from each input path between two output paths and impart aphase shift, i.e. 90° in a 2×2 3dB coupler, between signals on therespective output paths. The phase shift between the two output pathsdepends upon which input path the signal was introduced. Thus, thefrequency of the resulting distorted optical signal Λ_(OD) will beeither ν_(o±e)=ν_(o)+ν_(e) or ν_(o−e)=ν_(o)−ν_(e) depending upon whichinput of the coupler 56 the electrical signals are introduced.

Data encoding techniques, such as duobinary, QPSK, and others, areuseful to decrease the bandwidth of the resulting optical signal. Theseformats can also affect the extent of distortions that arise from signaldispersion and non-linear interaction between the signals. The detectionof duobinary and other differential PSK-type signals using directdetection can be enhanced using an optical filter 58 before the receiver14 in the optical system 10. The optical filter 58 can be matched, i.e.,comparably shaped, to the received optical spectrum of the signal, whichcan be controlled in the present invention using the electrical filter52. The optical filter 58 can be a Fabry-Perot filter or otherappropriate filter as may be known in the art. The electrical filter 52can be design to account for and match the properties of the opticalfilter 58 so as to minimize the bandwidth of the optical signal. It willbe appreciated that the electrical filter 52 can be positioned atdifferent locations within the transmitter 12 and modified accordingly.

In another aspect of the invention shown in FIG. 12, the transmitter 12of the present invention can be used to simultaneously upconvert aplurality of electrical signals Λ_(En) onto one optical carrier. Aplurality of baseband electrical signals Λ_(B1)-Λ_(Bn) are modulatedonto a corresponding plurality on electrical carriers provided bysources 42 ₁-42 _(n) to provide modulated electrical carriers. Signaldistorters 32 can be provided to distort either the baseband signal orthe modulated electrical carrier, if dispersion compensation is desired.The modulated electrical carriers are passed through the electricalcoupler 56, which divides the electrical signals between the two outputpaths leading to the upconverter 44.

Numerous combinations of electrical carriers can be upconverted usingthe transmitter configuration of FIG. 12. For example, electricalsources 42 ₁ through 42 _(n) can provide the same or differentelectrical carrier frequencies and depending upon how the carriers arecoupled into the upconverter 44. If more than two electrical carriersare to be upconverted using the same upconverter 44, the additionalcarriers can be combined, or multiplexed, onto the appropriate couplerinput. The resulting optical signal can be produced at longer or shorterwavelengths than the optical carrier wavelength λ_(o) as previouslydiscussed. In addition, it may also be possible to use one or moreelectrical subcarriers to carry additional data along with, or in lieuaof, data on the electrical carrier frequency depending upon theelectrical subcarrier frequency spacings.

The upconverter 44 in embodiments of FIGS. 12 and 13 is configured toupconvert the electrical signal onto a single sideband subcarrierfrequency, either ν_(o+e) or ν_(o−e), while suppressing the mirror imagesideband subcarrier frequency. The upconverter can be operated withoutor with carrier wavelength suppression, although carrier suppressioneliminates unwanted signals that could produce signal interference.

FIG. 14 shows an embodiment of the single side band suppressed carrierupconverter 44 suitable for use in the present invention. Other suitablesingle side band embodiments include those described by olshansky inU.S. Pat. Nos. 5,101,450 and 5,301,058, which are incorporated herein byreference. As shown in FIG. 14, the optical carrier lightwave atfrequency ν_(o) is split using an optical splitter 60 into tworespective optical paths, 62 ₁ and 62 ₂, which are further split intooptical paths 62 _(1′) and 62 _(1″). The split lightwaves in opticalpaths 62 ₁ are passed between first upconverter input electrode 64 ₁ anda pair of ground electrodes 66. Likewise, the split lightwaves inoptical paths 62 ₂ are passed between second upconverter input electrode64 ₂ and a pair of ground electrodes 66. Electrical input signals v₁ andv₂ are provided to the upconverter respective input electrodes 64 ₁ and64 ₂ via first and second inputs, 68 ₁ and 68 ₂, respectively. The inputsignals v₁ and v₂ are upconverted onto the respective split lightwavespassing between the electrodes and combined in cascaded opticalcombiners 70 to produce the upconverted optical signal Λ_(o).

In an embodiment, LiNbO₃ is used to form the optical paths 62 _(i′) and62 _(i″), which can be used to produce linearly polarized opticalsignals. In addition, bias electrodes can be provided in optical paths62 _(i′) and 62 _(i″) and/or 62 _(i) after passing through the inputelectrodes 64 ₁ and 64 ₂. The bias electrodes can be used to trim thephase difference of the optical signals upconverted onto the subcarrierlightwaves in each path before the signals are combined.

The electrical input signals v₁ and v₂ introduced to the upconverter 44carrying the same electrical data signal, except that the data signalshave a relative phase shift between the first and second inputs, 68 ₁and 68 ₂, according to the relation: v₁=v₂±phase shift. The sign of thephase shift determines whether the electrical data signal will beupconverted onto lightwave subcarriers that are greater or less than thecarrier frequency of the lightwave. Thus, the upconverter 44 can beconfigured to receive and simultaneously upconvert electrical signals atthe same or different electrical frequencies onto different subcarrierlightwave frequencies of the same lightwave by introducing theappropriate phase shift between the electrical input signals. Forexample, in embodiments of FIGS. 12 and 13, 3 dB electrical couplers 56provide a ±90° phase shift, which allows electrical signals to beupconverted onto optical frequencies that are greater or less than thecarrier frequency. One skilled in the art will appreciate that othertechniques for imparting the phase shift are suitable within the scopeof the invention.

The transmitter 12 shown FIG. 13 provides a configuration that can beused to symmetrically place two different optical signals around thecentral wavelength λ_(o) of the optical carrier. The electrical carrier42 supplies the electrical carrier ν_(e) that is split into two paths,each of which is modulated using a corresponding modulator 36 ₁ or 36 ₂with electrical baseband signals Λ_(B1) and Λ_(B2), respectively. Thetwo signals are passed through the electrical coupler 56 which splitsand couples the signals from each of the two coupler input paths to eachof the two output paths. The coupler 56 introduces a 90° phase shiftinto the coupled portion of the signal, shown as Λ_(e1) ^(P) and Λ_(e2)^(P) on FIGS. 12 and 13, to produce upconverter input signals v₁ and v₂.For example in FIG. 13, v₁ includes Λ_(e1) ^(P) and Λ_(e2) ^(P), whereasv₂ includes Λ_(e1) and Λ_(e2) ^(P). The opposite phase shifts of v₁ andv₂ results in one of the two electrical signals being upconverted ontoan optical subcarrier frequency v_(o+e) and the other electrical signalis upconverted onto the optical subcarrier frequency ν_(o−e), symmetricto the optical carrier frequency ν_(o). A skilled artisan will recognizethat distorted and undistorted optical signals can be produced using theembodiment of FIG. 13 and similar embodiments.

An embodiment of the transmitter 12, shown in FIG. 15, can be also usedto provide control over proximate optical wavelengths by upconvertingone or more electrical frequencies onto a plurality of optical carriersprovided by the common optical source 34. The optical carrier lightwaveis split using the optical splitter 60 into split lightwaves carried ona plurality of optical paths 62 ₁-62 _(n). A corresponding plurality ofthe upconverters 44 _(1−n) are disposed along the optical paths. Aplurality of electrical baseband signal Λ_(B1)−Λ_(Bn) arecorrespondingly modulated onto electrical carrier ν_(e1)-ν_(en) viamodulators 40 _(1−n). The electrical carrier signals Λ_(e1)-Λ_(en) areprovided to the upconverters 44 _(1−n) and converted to subcarrierlightwave optical signals Λ_(o1)-Λ_(on) at frequencies ν_(oe1)-ν_(oen)and combined using an optical combiner or multiplexer 68. When only oneelectrical signal is upconverted onto a split lightwave optical carrierin a path 62 _(i), single or double sideband upconverters, with orwithout carrier suppression, can be used in the invention. Opticalfilters 58 can be employed to remove any undesired remnant carrierwavelengths or mirror image sidebands that are output from theparticular modulator used in the transmitter 12.

FIG. 16 shows an embodiment of the transmitter 12 that is configured totransmit four optical signals using a single optical source 34, such asa laser 72, emitting the optical carrier at a central wavelength λ_(o)and frequency ν_(o). The baseband electrical signal Λ_(B1)-Λ_(B4) areprovided as input to corresponding data encoders 50 ₁₋₄ from anelectrical transmission path or from the optical receiver 14 in a shortor long reach optical system. The encoded electrical signal is passedthrough the shaping filter 52 ₁₋₄ to respective electrical modulators40. Encoded electrical signals Λ_(C1)-Λ_(C2) and Λ_(C3)-Λ_(C4) aremodulated onto the electrical carrier at frequency ν_(e1) and ν_(e2),respectively. The modulated electrical signals Λ_(e11)-Λ_(e24) arepassed through respective signal distorters 32 ₁₋₄ and electricalamplifiers 54 ₁₋₄ to provide amplified distorted electrical signalsΛ_(e11D)-Λ_(e24D). Electrical signals Λ_(e11D) and Λ_(e23D) can berouted through electrical coupler 56 ₁ to upconverter 44 ₁. Likewise,electrical signals Λ_(e12D) and Λ_(e24D) can be routed throughelectrical coupler 54 ₂ to upconverter 44 ₂ The upconverted opticalsignals Λ_(oe1D)-Λ_(oe4D) are combined in the combiner 62 prior totransmission. The interleaving of the electrical frequencies beingupconverted allows for the use of optical filters 58, with either singleor double sideband modulators, to remove any unwanted sidebands orcarrier wavelengths from the optical signals Λ_(oe1D)-Λ_(oe4D).Transmitters 12 of the present invention can also be used to modulatedata onto the lightwave carrier wavelength, in addition to upconvertingelectrical frequency onto the lightwave.

In the present invention, transmitters 12 configured to provide multipleoptical signals, can be further configured to impart oppositepolarization to pairs of optical signals being generated by upconvertingthe electrical signals. For example, the optical combiner 62 inembodiments such as those shown in FIGS. 15 and 16 can be a polarizingcomponent, such as a polarizing beam splitter/combiner. The orthogonalpolarization of adjacent signals will reduce or eliminate nonlinearinteraction between the signals, thereby providing for more closelyspaced signal wavelengths and high power signals.

Alternatively, as shown in FIG. 17, a separate polarizing element 74 canbe included in the combiner 62. An embodiment of the polarizing element74 can includes two oppositely configured polarizing beam splitters 76connected in series by two parallel paths 78 that produce a differentialtravel time between the splitters 76. The first beam splitter 76 splitsthe optical signal into two equal amplitude polarization components. Thesecond beam splitter 76 is used to recombine the two polarizationcomponents. The time differential introduced by the parallel paths 78can be established and/or controlled to introduce differences in thepolarization of the channels. For example, optical signals havingsufficiently narrow bandwidths can be introduced to the first beamsplitter 76 at a 45° polarization angle to allow optical signal power topropagate equally in both paths 78. The resulting combined signalsemerging from the second splitter 76 would be orthogonal if the timedifferential were equal to 1/(2*frequency difference between thesignals). Similarly, polarization maintaining fiber can be used in lieuof the splitters 76 and parallel path 78 to introduce the timedifferential between the polarization components of a linearly polarizedoptical signal.

It will be appreciated that the present invention provides for opticalsystems having increasing the number of channels and the transmissionperformance of optical systems. Those of ordinary skill in the art willfurther appreciate that numerous modifications and variations that canbe made to specific aspects of the present invention without departingfrom the scope of the present invention. It is intended that theforegoing specification and the following claims cover suchmodifications and variations.

What is claimed is:
 1. An apparatus, comprising: an optical carrier source; an optical splitter connected to the optical carrier source and having a plurality of outputs; a plurality of optical upconverters, each having an optical input connected to a corresponding output of the optical splitter, each having at least one electrical signal input, and each having an optical output, wherein each optical upconverter receives at least one electrical signal indicative of at least one data signal and each optical upconverter upconverts a corresponding at least one data signal onto at least one optical sideband of a split optical carrier, and wherein the at least one data signal is different for each optical upconverter; and an optical combiner having a plurality of inputs, each connected to an optical output of a corresponding one of the optical upconverters.
 2. The apparatus of claim 1, wherein each of the optical upconverters is responsive to a single electrical oscillator and wherein no two optical upconverters are responsive to the same electrical oscillator.
 3. The apparatus of claim 1, wherein the at least one electrical signal is indicative of a single baseband data signal.
 4. The apparatus of claim 1, wherein the optical upconverters are single Mach-Zehnder upconverters, each having a single electrical signal input, and each producing double sideband outputs.
 5. The apparatus of claim 1, wherein the optical upconverters are double Mach-Zehnder upconverters, each having two electrical signal inputs.
 6. The apparatus of claim 5, wherein the optical upconverters produce single sideband outputs.
 7. The apparatus of claim 5, wherein the optical upconverters suppress the split optical carriers.
 8. The apparatus of claim 1, further comprising at least one optical filter between the optical combiners and at least one of the optical upconverters.
 9. The apparatus of claim 1, further comprising: a plurality of electrical oscillators corresponding to the plurality of optical upconverters; and a plurality of electrical modulators, wherein each electrical modulator has a electrical oscillator input connected to a corresponding electrical oscillator, each has an modulated signal output connected to an electrical signal input of a corresponding one of the optical upconverters, and each has an electrical data input.
 10. The apparatus of claim 9, wherein each of the electrical oscillators has a different oscillation frequency.
 11. The apparatus of claim 9, further comprising a plurality of receivers, each having an electrical output connected to an electrical data input of a corresponding one of the electrical modulators.
 12. The apparatus of claim 9, wherein the receivers are optical receivers, and wherein the receivers convert optical signals to electrical signals.
 13. The apparatus of claim 12, further comprising an optical filter before the receiver.
 14. The apparatus of claim 11, further comprising at least one electrical filter connected between the receivers and the electrical modulators.
 15. The apparatus of claim 14, further comprising at least one data encoder connected between the receivers and the electrical modulators.
 16. An apparatus, comprising: an optical carrier source; an optical splitter connected to the optical carrier source and having a plurality of outputs; a plurality optical upconverters, each configured as a double Mach-Zehnder upconverter, each having an optical input connected to a corresponding output of the optical splitter, each having at least one electrical signal input, and each having an optical output, wherein the plurality of upconverters are configured to upconvert at least two different data signals onto separate optical frequencies; and an optical combiner having a plurality of inputs, each connected to an optical output of a corresponding one of the optical upconverters.
 17. The apparatus of claim 16, wherein each data signal is upconverted onto a single optical frequency, which is separate from optical frequencies onto which other data signals are upconverted.
 18. The apparatus of claim 16, wherein each data signal is upconverted onto a pair of optical frequencies, where each optical frequency is separate from optical frequencies onto which other data signals are upconverted.
 19. The apparatus of claim 16, wherein the plurality of optical upconverters suppress a carrier signal provided by the optical carrier source.
 20. An apparatus, comprising: first, second, and third electrical carrier sources having first, second, and third electrical frequencies, respectively, wherein none of the first, second, and third electrical frequencies are equal to the other electrical frequencies; first, second, and third electrical modulators connected to the first, second, and third electrical carrier sources, respectively, each having a data input and each having an output; an optical carrier source having an output; an optical splitter connected to the optical carrier source and having first, second, and third outputs, each for providing a split carrier source; first, second, and third optical modulators, each having an optical input connected to a corresponding one of the first, second, and third outputs of the optical splitter, each having an electrical input connected to a corresponding one of the first, second, and third electrical modulators, and each having an optical output; and an optical combiner having first, second, and third inputs, each connected to a corresponding one of the outputs of the first, second, and third optical modulators. 